Magnetic resonance imaging is presently used in many cases as one of imaging methods for medical use. Magnetic resonance imaging is an imaging method that gives rise to magnetic excitation of nuclear spins in a subject to be imaged positioned in a static magnetic field with a high frequency signal at the Larmor frequency and then reconstructs an image of the internal of the subject to be imaged using an MR signal induced in association with the excitation. Magnetic resonance imaging includes various types, and the type is also divided according to pulse sequences used for magnetic excitation and signal acquisition.
In the case of magnetic resonance imaging for imaging a region in the heart, ghost-like artifacts (blood flow artifacts) readily appear on a reconstructed image in a phase-encoding direction from a portion where a number of blood flows are present due to influences of a pulse beat of blood. In order to suppress the artifacts, the cardiac synchronization imaging method is generally used, by which RF excitation and echo acquisition are synchronized with electrocardiographic waveforms. According to this method, a variance of an echo signal occurring in each shot (excitation) can be suppressed, and the aforementioned blood flow artifacts can be thereby reduced.
Also, as one can review in articles, Edelman RR et al., “Fast selective black blood MR imaging”, Radiology 1991 December 181(3): 655-60, 1991, Edelman RR et al., “Extracranial carotid arteries: evaluation with “black blood”MR angiography”, Radiology 1990 October 177(1): 45-65, 1990, etc., with the aim at chiefly improving a capability of extracting the cardiac muscle, a so-called black blood method has been proposed, by which a pre-pulse used to suppress the acquisition of an MR signal of blood is appended to the front of a pulse train for normal RF excitation and echo acquisition. As for a pulse train for the RF excitation and the echo acquisition, a pulse train through the field echo method, the fast field echo method, the fast spin echo method, etc. is used as the pulse train.
Moreover, there has been recently reported the black blood method using a so-called double inversion pulse, by which a selective inversion pulse that gives rise to inversion excitation in a region substantially the same as the imaging plane and a non-selective inversion pulse that gives rise to inversion excitation in the whole subject to be imaged are successively applied, and the RF excitation and the echo acquisition for imaging are performed 400 to 700 ms later from this application (see articles, for example, Simonetti OP et al., ““Black Blood” T2-weighted inversion-recovery MR imaging of the heart”, Radiology 1996 April 199(1): 49-57,1996, Stehling MK et al., “Single-shot T1- and T2-weighted magnetic resonance imaging of the heart with black blood: preliminary experience”, MAGMA 1996 September December, 4(3-4): 231-40, 1996, Arai AE et al., “Visualization of aortic valve leaflets using black blood MRI”, J Magn. Reson. Imaging 1999 November, 10(5): 771-7, 1999, etc.)
The black blood method using the double inversion pulse attracts the attention because of its advantages that a suppressing effect of a blood signal is high and deterioration in signals of other tissues is small, and is expected to become increasingly more popular. The shape of the pre-pulse varies from report to report. Because the pre-pulse is basically intended to reduce the longitudinal magnetization of a blood signal to a null point or to a sufficiently small level, a time from the application of the pre-pulse to the application of an excitation pulse for imaging is approximately 400 to 700 ms in all the reports, which is longer than in the case of a pre-pulse applied for other purposes.
For this reason, even when the pre-pulse is applied immediately after the detection of an R-wave through the use of the cardiac synchronization imaging method, cardiac temporal phases that can be actually imaged are those in a time zone in the latter half of the cardiac cycle, that is, in the diastole. Hence, as one can review in the article supra, Simonetti Op et al., ““Black Blood” T2-weighted inversion-recovery MR imaging of the heart”, Radiology 1996 April 199(1): 49-57, 1996, it is general to acquire images in the diastole when this black blood method is used.
A pulse sequence of the conventional black blood method using the double inversion pulse is shown in FIG. 1. As shown in the drawing, a double inversion pulse DIV as the pre-pulse for blood suppression is applied in sync with an ECG (electrocardiogram) signal with a predetermined time td from an R-wave thereof, then an imaging pulse train SEQima is applied when a predetermined waiting time BBTI has elapsed since this application, where by echo signals are acquired. In the drawing, RF indicates an RF pulse, Gs indicates a slicing direction gradient magnetic field, Gr indicates a readout direction gradient magnetic field, Ge indicates a phase-encoding direction gradient magnetic field, and Echo indicates an echo signal.
Of the two RF pulses of the double inversion pulse DIV, one is applied with the slicing direction gradient magnetic field Gs of a zero strength, and the other is applied with the slicing direction gradient magnetic field Gs of a necessary strength in order to give rise to excitation in a region same as the slice subjected to selective excitation with a pulse sequence for imaging. The waiting time BBTI is approximately 500 to 600 ms in general, and is set to a time at which the longitudinal magnetization of blood is reduced to or nearly to a null point.
However, when the conventional black blood method is used, there arise problems that images in the systole near an R-wave on time are difficult to acquire, and it is difficult to acquire a series of images having latencies that vary at regular intervals as with a cine mode display.
Because a long time:BBTI (inversion time of the black blood method) is needed from the application of the pre-pulse to the echo acquisition, in order to acquire images in the systole, an R-wave in the last or earlier cardiac cycle with respect to a cardiac cycle from which images are actually acquired has to be used as a synchronous trigger. This state is shown in FIG. 2. That is, as shown in the drawing, in order to acquire images in the systole from R2 to R3, synchronization has to be made with an R-wave:R1 with a predetermined delay time td in the earlier cardiac cycle, for example, in the cardiac cycle from R1 to R2.
The cycle of heartbeats varies by 10 to 20% even in a normal healthy subject. That is, the position of an R-wave is displaced on the time axis in every heartbeat. Hence, even when images are acquired after a certain time “td+BBTI” from an R-wave in the last or earlier cardiac cycle, the position of the cardiac muscle at the time instance of the echo acquisition sways in each shot, which deteriorates the image quality so badly that it is impossible to obtain an image that can be used for a diagnosis. FIG. 3A and FIG. 3B are views schematically showing FIG. 2 divided by two conditions of a long cardiac cycle (FIG. 3A) and a short cardiac cycle (FIG. 3B). As shown in FIG. 3A and FIG. 3B, in a conventional case, the start timing of an imaging pulse train SEQima is controlled by fixing a delay time td1 and an inversion time BBTI, which causes a delay time td2 having a strong correlation with actual motions of the cardiac muscle to vary in the same manner as the cardiac cycle varies.
For this reason, as has been described above, it is the images in the diastole alone that can be acquired in the conventional method.
The invention was devised to break through the current situation of the foregoing related art, and therefore, has an object to provide an imaging method capable of capturing images in the systole of the cardiac cycle in a reliable manner, even in the presence of a cycle-to-cycle variance of the cardiac cycle, in MR imaging using a pulse sequence in which a waiting time until the application of an imaging pulse train after a pre-pulse was applied is relatively long in comparison with the cardiac cycle like the imaging through the black blood method using a double inversion pulse.